Module exposure and privacy

[fmease]

• [x] plain @public
• [x] restricted exposure via @(public scope) with scope variable
• [x] rules around punctuation binders: public (even in restricted forms) punctuation must have a public alphanumeric alias disallow any re-exports of private bindings
• [x] public constructors
• [x] @opaque on data declarations (private constructors)
• [x] good diagnostics
• [x] thorough test suite
• [x] (optional, otherwise future work) private-in-public linting (prototype, later becoming a standard error instead of a denied lint) moved to #70

[fmease]

1. Basic implementation in 17acdbf215c9f126363a563622b9d433daacae06.
2. Improved some diagnostics in 9e58826431f177b2d7c321c94a8001ce16736dea
3. Disallow re-exports of more private bindings in de3f636e89b8c214898fce4d537a88726d4511d5
4. Add thorough (!) test suite and improve some diagnostics as well in 3185be953ca38d9e59dd2125b3d37fe41b36aa0b